The quadrupole ion trap (QIT) was first disclosed in the year 1952 in a paper by Paul, et al. This 1952 paper disclosed the QIT and the disclosure of a slightly different device which was called a quadrupole mass spectrometer (QMS). This quadrupole mass spectrometer was very different from all earlier mass spectrometers because it did not require the use of a magnet and because it employed radio frequency fields for enabling the separation of ions, i.e. performing mass analysis. Mass spectrometers are devices for making precise determination of the constituents of a material by providing separations of all the different masses in a sample according to their mass to charge ratio. The material to be analyzed is first disassociated/fragmented into ions which are charged atoms or molecularly bound group of atoms.
The principle of the quadrupole mass spectrometer (QMS) relies on the fact that within a specifically shaped structure, radio frequency (RF) fields can be made to interact with a charged ion so that the resultant force on certain of the ions is a restoring force thereby causing those particles to oscillate about some referenced position. In the quadrupole mass spectrometer, four long parallel electrodes, each having a highly precise hyperbolic cross sections, are connected together electrically. Both dc voltage, U, and RF voltage, V.sub.0 cos.omega., can be applied across the electrodes. When an ion is introduced or generated within the spectrometer, if the parameters of the quadrupole are appropriate to maintain the oscillation of those ions, such ions would travel with a constant velocity down the central axis of the electrodes at a constant velocity. Parameters of operation could be adjusted so that ions of selected mass to charge ratio, m/e, could be made to remain stable in the direction of travel while all other ions would be ejected from the axis. This QMS was capable of maintaining restoration forces in two directions only, so it became known as a transmission mass filter. The other device described in the above mentioned Paul, et al. paper has become known as the quadrupole ion trap (QIT). The QIT is capable of providing restoring forces on selected ions in all three directions. This is the reason that it is called a trap. Ions so trapped can be retained for relatively long periods of time which supports separation of masses and enables various important scientific experiments and industrial testing which can not be as conveniently accomplished in other spectrometers.
The QIT was only of laboratory interest until recent years when relatively convenient techniques evolved for use of the QIT in a mass spectrometer application. Specifically, methods are now known for ionizing an unknown sample after the sample was introduced into the QIT (usually by electron bombardment), and adjusting the QIT parameters so that it stores only a selectable range of ions from the sample with the QIT. Then, by linearly changing, i.e. scanning, one of the QIT parameters, it became possible to cause consecutive values of m/e of the stored ions to become successively unstable and to sequentially pass the separated ions which had become unstable into a detector. The detected ion current signal intensity, as a function of the scan parameter, is the mass spectrum of the trapped ions.
The first step in every analysis of a sample in a QIT employs ionization. We have determined that an improved mass range isolation during ionization procedure can be of significant benefit in analysis.
It was recognized in the prior art that it was beneficial to reduce the range of ions retained in a QIT during ionization. The European patent 0362,432 of Franzen provides a so called supplemental broadband RF excitation voltage to the end caps of the trap during the electron bombardment ionization. The broadband voltage was to be designed to contain frequencies corresponding to the secular frequencies of all the unwanted ions that were in the trap. The intention was that the unwanted ions would absorb power from such selected frequency components and increase their secular motion and be ejected or removed by impacting the trap. Marshall, et al. patent, U.S. Pat. No. 4,761,545, teaches the application of a supplementary broad-based RF excitation signal applies to the end caps for ejecting ions where the broadband RF excitation signal is generated by an inverse Fourier Transform computation. It is also known to create the broadband waveform as disclosed in the Kelly U.S. Pat. No. 5,134,286, where filtered noise is selected to provide a waveform for exciting and ejecting the unwanted ions.
There are several disadvantages with the above-mentioned processes for ion range selection during ionization. The processes which require use of a frequency component which matches the frequency of each unwanted ion implies the knowledge of the precise secular frequency for each such ion. There are several practical reasons why this knowledge is extremely difficult, if not impossible, to obtain. The mass of an ion is not exactly an integer value and will have a "mass defect" causing its exact mass to differ from its nominal integer mass. Also, space charge effects, electronic drifts in the RF voltage and applied supplemental frequency cause mismatches, as well as physical trap imperfections. In respect to the Kelly continuum noise spectra, although this guarantees that the broadband excitation includes frequencies to match the required secular frequency, the amplifiers need to provide very high power because it is necessary to provide power at all frequencies in the noise continuum.
Furthermore, a large percentage of the power required by the Kelly technique is not used since in the lower mass range there are large differences between the secular frequency of adjacent masses. In the abstract of the Kelly U.S. Pat. No. 5,256,875, these supplemental waveforms are described as "a filtered noise signal having no missing frequency components outside of the notches of the notch filter employed to generate the filtered noise signal." This supplemental waveform is further described in the Kelly specification at column 12, line 51-58 for waveforms constructed of discrete frequencies as "The frequencies of the optimized broadband signal frequency components should be sufficiently close so as to present a substantially continuous band of frequencies to that physical system. In the embodiments of the previous paragraph, this implies that the separation df should be sufficiently small that the broadband signal presents a substantially continuous band of frequencies to the physical system."
The power inefficiency of the Kelly technique can be understood by considering the number of unused components. If an ion of m/e=32 has a secular frequency of 485 KHz, the next larger mass, m/e=33 would have a secular frequency of 429 KHz. Assuming a .DELTA.f=250 Hz in the noise waveform, there would be 224 component frequencies required between two adjacent masses. Since the range of secular frequencies typically spans 500 KHz to 10 Kz, many thousands of frequency components are dictated which would not correspond to an ion resonance.
The Franzen method (EP 362432) requires much fewer frequencies. If it were possible to exactly determine the secular frequencies, i.e. compensating for the mass defects, space charge shifts and other frequency error sources, the Franzen technique, for the mass range 10-650, would require at most one frequency for each unwanted mass to be ejected or 640 frequency components. However, since the frequency spacing of the secular frequencies of larger masses are very close, i.e. the difference between the secular frequency of m/e=400 and m/e=401 equals 100 Hz, it is clear that one supplemental frequency component can excite several adjacent secular frequencies for higher mass ions and the total number of frequency components in a frequency optimized waveform is much less than the previously mentioned 640. There is still another significant inefficiency when using frequency spacing of the frequency components in the supplemental waveform. Close spacing (df) will result in several different frequency components, with different phases, driving the same ion. A single ion driven by more than one external frequency will result in a complex beating in the amplitude of oscillation. Since the ion is to be ejected by means of increasing its amplitude until it strikes an electrode, an additional time or power is expended while undergoing a complex oscillation its amplitude before ejection.